It is common during offset printing to provide some means for preventing ink transfer from one sheet to another during stacking of the sheets following printing of them. In smaller size printing presses, such means customarily consists of a spray assembly that sprays the sheets passing to the collection station with an anti-offset material such as an inert powder. The powder spaces adjacent sheets from each other as they are stacked at the collection station. Electrical control means may be provided for causing the spray assembly to operate cyclically such that the anti-offset material is dispensed only when it will impact upon a freshly printed sheet, and not at those times when it would pass between successive ones of the sheets moving along the path of travel to the collection station. This minimizes the amount of anti-offset material used during operation of the press, which is desirable both from the viewpoint of lessening the amount of material that must be purchased, and also from the viewpoint of reducing the time that must be devoted to vacuuming or otherwise cleaning surplus material from the press.
In larger size presses, and more recently in some of the smaller ones, infrared or other sheet heating assemblies are used to dry the ink upon the freshly printed sheets, and to thus prevent or at least lessen ink transfer between them at the collection station. Vendors and other proponents of the sheet heating assemblies stress that, unlike the powder spraying assemblies, they do not require periodic refilling and press vacuuming or similar cleaning. On the other hand, proponents of the spraying assemblies point out that the heating assemblies require substantial amounts of power and have the potential for creating fires. Due to these mutually exclusive viewpoints, and also due in part to the limited amount of space available in smaller offset printing presses for mounting an anti-offset assembly of any type, presses possessing an assembly of either of the aforesaid types customarily have not also had an assembly of the other type. However, in different situations the prevention of ink offset may be best achieved by applying heat to the freshly printed sheets, or by applying powder to them, or by applying both heat and powder. By way of illustration in the latter regard, the application of both limited heat and limited powder to the sheets might be desirable when the prevention of ink offset by the use of heat alone would require excessive power, or operation of the press at an undesirably slow speed.